Random access over wireless links: optimal rate and activity probability selection
Author
Karakoç, Nurullah
Advisor
Duman, Tolga Mete
Date
2017-07Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Due to the rapidly increasing number of devices in wireless networks with the
proliferation of applications based on new technologies such as machine to machine
communications and Internet of Things, there is a growing interest in the
random access schemes as they provide a simple means of channel access. To
this end, various schemes have been proposed based on the ALOHA protocol to
increase the e ciency of the medium access control layer over the last decade.
On the other hand, physical layer aspects of random access networks have received
relatively limited attention, and there is a need to consider optimal use of
the underlying physical layer properties especially for transmission over wireless
channels.
In this thesis, we study uncoordinated random access schemes over wireless
fading channels where each user independently decides whether to send a packet
or not to a common receiver at any given time slot. To characterize the system
throughput, i.e., the expected sum-rate, an information theoretic formulation is
developed. We consider two scenarios: classical slotted ALOHA, where no multiuser
detection (MUD) capability is available and slotted ALOHA with MUD. Our
main contribution is that the optimal rates and the channel activity probabilities
can be characterized as a function of the user distances to the receiver to maximize
the system throughput in each case (more precisely, as a function of the average
signal to noise ratios of the users). We use Rayleigh fading as our main channel
model, however, we also study the cases where log-normal shadowing is observed
along with small scale fading. Our proposed optimal rate selection schemes o er
signi cant increase in expected system throughput compared to the same rate
approach commonly used in the literature. In addition to the overall throughput optimization, the issue of fairness among users is also investigated and solutions
which guarantee a minimum amount of individual throughput are developed. We
also design systems with limited individual outage probabilities of the users for
increased energy e ciency and reduced delay. All of these analytical works are
supported with detailed numerical examples, and the performance of the proposed
methods are evaluated.
Keywords
Random accessRayleigh fading
Shadowing
Machine to machine communications
Multiple access channel
Channel capacity
ALOHA networks
Multi user detection
Throughput
Fairness